What The Hell Is Compression And What Does It Do?

Few effects cause more confusion, and are more often misused, than compression. Good compression can bring a mix to life, solve all sorts of audio problems, and even improve your live performances. Compressing the wrong way can ruin any chances of whatever you're working on sounding acceptable, and is a sure-fire way to ruin even the best of mixes.
I was recording and mixing for years without really understanding compression or how it worked, and as a result whether what I came out with sounded good was a matter of trial and error, or even blind luck. Compression is almost infinitely complex, but I've gained a good understanding of it over the years (though I'm still learning), so I'm going to take you through the basics to save you the guesswork I had to go through.

The Basics

Let's start with first principles. The most important thing to understand, and to bear in mind at all times when working with compression, is that compression always makes loud things quiet. This may seem a little counter-intuitive, as we often associate compression with loudness, but when compression is used to make things seem louder it is not in fact the compressor that is doing that more on that in a bit.
Compression works by taking the loudest parts of the waveform it is assigned to and reducing the gain, bringing it more into line with the quieter parts of the waveform. The controls that come with compressors help you decide what parts of the waveform you want made quieter, how much quieter you want them to be, and when you want that to kick in.

What is it used for?

There are three main reasons that you will want to use compression. One is to even out a sound that is uneven in its volume levels. Vocals, in particular, will almost always need compression to sound consistent, as the wide range of sounds made by the human mouth have a wide variation in volume, and singers have this annoying habit of moving around, varying the distance between their lips and the mic in a most inconsiderate fashion. By compressing the louder parts down, the producer (or the sound guy in a live setting) can set a level for the vocals and leave it there, knowing that things won't get too loud or quiet. Other instruments that can benefit from this include acoustic and bass guitars, as well as drums.
The second, related use is to increase loudness. Because compression reduces the peak levels of the signal, you can increase the gain on a live or recorded track without causing overdrive or distortion. That way, rather than just the hardest-hit chords or biggest drum hits being the highest peaks in perceived volume, with enough compression everything can be made deafeningly loud without anything peaking or distorting. This is particularly noticeable in modern mastering techniques, whereby, to compete in the so-called loudness wars, engineers make everything near or at as loud as it can be. This has certainly been overdone on occasion, listen to the Foo Fighters' All My Life or the Deftones' Minerva for examples of over-compressed mastering. Both those songs have quiet introductions and burst into life when the drums come in, but because the main parts of those songs have been compressed down to match the level of the intros, and then had everything turned up to 11, those moments don't have as much impact in fact it almost seems as if everything gets quieter when the full band comes in, the exact opposite of what you'd expect and of what you'd experience in a live setting. Listen to Drowning Pool's Bodies to hear more sympathetic mastering which enables the band to really punch the listener in the face when they come in.
Compression for loudness is often used live, especially for bass guitar. A slap/pop bassline will have huge peaks as the strings slap off the frets, and without compression you'd have to set the levels for the bass guitar unacceptably low just to prevent the pops peaking and possibly blowing the speakers. With compression, those peaks are reigned in and you can turn up the volume so that the quieter parts of the bassline are audible as well.
A third use of compression is to make tonal changes, especially to drums (and sometimes acoustic guitar), by manipulating the percussive attack (the transient) of each hit (or strum). More on that below.

What do the controls do?

Every compressor will have different controls, but the essential elements of every compressor are:
Threshold: This is the level (counting down from 0dB as the maximum possible extent of the waveform) at which the compressor will start to reduce gain. Set it high, and the compressor will only quieten the very loudest peaks. Set it low, and the compressor will cut into even medium-gain peaks and reduce the gain of much more of the waveform in effect, it will make the quieter sounds in the waveform (those below the threshold) sound louder rather than make the loud ones seem quieter.
Ratio: This is the extent to which the peaks above the threshold will be reduced. Set at infinity, the compressor will not allow any sound that is louder than the threshold to go past this turns the compressor into a limiter. Limiting can cause distortion if it is cutting too deeply into the tops of the waveforms, though some plugins, especially specialist limiters, have features designed to reduce this. Otherwise, the ratio determines how squashed the signal above the threshold should be, ie a ratio of 3:1 means that a peak above the threshold of 3dB would be reduced to 1dB etc.
Soft Knee: Some compressors have a setting that tells the compressor to start gently compressing before the threshold is reached. This helps reduce distortion and other artefacts from high-ratio compression or limiting, at the expense of slightly greater gain reduction and slightly reduced loudness. Some compressors, particularly vintage or analogue-style compressors, do this automatically as part of the way they work.
Makeup Gain: As discussed at the start, compression makes things quieter, but one of its chief uses is to make things louder. To achieve this, most compressors have a makeup gain control which lets you add gain after the compression stage to achieve the compression/loudness process all in one plugin. Most such compressors let you either choose the amount of gain you add or have an auto setting to bring it back to the average or peak gain before compression.
Attack/Release: These controls set the length of delay between a signal passing the threshold and being compressed, and the length of time that the compression lasts for. These can make big tonal differences to the sound created, and in particular, are the most important controls when using compression to manipulate transients. Careful use of the attack and release controls lets you either emphasise or reduce the attack of instruments (or, looking at it another way, emphasising or reducing the sustain) of instruments with strong and distinctive attack characteristic, notably drums, but also acoustic guitar and bass. The process of how to achieve this is quite difficult without audio examples, so I've created a short audio guide to doing this. Sign up to my newsletter and download it for free.
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About the Author:James Scott is a London Music Producer, writer and session musician.

I've got a question here...
1. Compression applies to guitars as a rhythm track to get an even tone?
2. What about solos when you want to increase the dynamics, does the compressor still work or will it end up balancing the "peak" and soft" part of the solo?

EpiExplorer wrote:
Very throrough, although also do something on limiters, there's a lot of confusion between the two.

At heart, a limiter is a compressor with a very high ratio. As the name says, they essentially limit the signal from going beyond a certain db threshold.
Some limiters apply the compression incrementally as the signal approaches the threshold, which reduces distortion (also called 'hard knee' or 'soft knee' compression).
They're essentially the same device, just used for slightly different purposes.

Tallica9000 wrote:
Beauty, I always figured that compressors actively boosted the quietest parts of the waveform as well as shrank the loudest ones, but it looks like it's only the latter.

Yep.
It's the makeup gain which boosts the reduced signal in a classic compressor.
Although some compressor designs have just one threshold control and no gain, and it really is dependent on the model as to how they function. Some boost the signal against a threshold compression 'wall' and make the sound louder in that way.

ThatGuy_17 wrote:
I've got a question here...
1. Compression applies to guitars as a rhythm track to get an even tone?
2. What about solos when you want to increase the dynamics, does the compressor still work or will it end up balancing the "peak" and soft" part of the solo?

1. That's a very effective use for it.
2. Compression reduces dynamics, but it depends on where in the signal chain you put the compressor as to how much it effects the tone. Before the amp/distortion, it will result in a more even tone. After the amp will result in amore even volume level, but the dynamics of the guitar going into the amp are preserved, so the 'biting' sound will be there.

The majority of this is great for beginners, and the article is well-written, but there are a few flaws I have to take issue with:
1) Your second 'use' of compression is a bit inaccurate - when talking about compression artifacts, you mistakenly claim compression can make things louder without any distortion. This is not true, as compression of any nature distorts the waveform by altering the shape as it restricts the rise in level beyond the threshold. This distortion is just the same as a vacuum tube smoothing the waveform as it passes beyond the clean headroom and into overdrive, though will inevitably sound different depending on the way the compressor acts.
2) When discussing the 'Threshold', it's important to note that 0dB is only the theoretical maximum of the digital realm - analogue gear often passes signal well beyond 0dB as this is a different reference/measure of dB and does not have the same limitations as digital's dBFS (Decibels Full-Scale). You can therefore, theoretically (as it is not particularly useful), have analogue compressors that can set a threshold above 0dB allowing the waveform to eat into the device's non-clean headroom (analogue tape acts in this way, when pushed beyond 0dB).
3) I also think your definition of release times is a bit too simplified - it isn't necessarily 'the time the compression lasts for', but the time taken for the compressor to stop compressing once the signal has dropped back below the threshold level . One could mistake your definition for meaning the compressor will be triggered and then start to release immediately, or that release time is a variable based on how long the signal is above the threshold, when in reality, release time is measured from the point the signal drops below the threshold, to the point that the compressor is no longer affecting the level of the signal.
Sorry to nitpick though - otherwise a great article and I can see it helping people understand compression when they are new to the art!

This is an extremely useful article for me. I've been trying to figure out why my recordings stunk so badly, especially the vocals. This makes a whole bunch of sense, besides the fact that I'm still working to learn to use my old Tascam 8 track. Thanks